Since the report on the ionic conductivity of a poly(ethylene oxide)--alkali metal salt complex, there has been considerable interest in ion conducting polymers. Particular attention has focused on the possibility of using such polymers as components of solid electrolytes in high energy density batteries, such as lithium batteries as reported by M. B. Armond et al., "Fast Ion Transport in Solids", North Hollow, N.Y. (1979). In comparison with the electrolytes based on polar aprotic organic liquids that are conventionally used in such batteries, as disclosed, for example, by G. Blomgren in "Lithium Batteries", Chapter 2, Academic Press (London) 1983, it has been proposed that polymer-based electrolytes might offer some or all of the following advantages:
Increased safety
Lower vapor pressure and enhanced thermal stability
Reduced corrosion and lower reactivity with active electrodes such as lithium.
Superior mechanical properties, such as dimensional stability and/or compliance, that yield handling and mechanical advantages in processing and manufacture, and increased durability in use.
The reduction or elimination of electrolyte leakage
In seeking to find polymer-based electrolytes that embody some or all of the potential advantages listed above, and with ambient temperature conductivities approaching those of the conventional liquid electrolytes, investigators have surveyed a large number of suitable organic monomers, preferably containing at least one polar group or atom capable of complexing with the cation(s) of the electrolyte salt(s) (e.g. alkali metal ions). When polymerized, these compounds form polymers suitable for use in electrolyte compositions. Suitable organic polymeric matrices that are generally known in the art are organic homopolymers obtained by the polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283 or copolymers obtained by polymerization of a mixture of organic monomers. Suitable organic monomers or polymers include, by way of example, ethylene oxide, propylene oxide, ethyleneimine, polyepichlorohydrin, poly(ethylene succinate), and an acryloyl-derivatized poly(alkylene oxide) containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl having from 1 to 6 carbon atoms.
Polymers proposed for use in polymer electrolytes, and more specifically as the basis for solid polymer electrolytes, are described in many publications. A number of such electrolytes are described in papers presented at the Third Int. Symposium on Polymer Electrolyte, reported in Electrochim. Acta 37(9), 1992 ed. M. Armand and A. Gandini. Polymers used have ranged from polyethers such as PEO or PPO to comb polymers, for example having backbones comprising highly flexible polyphosphazenes to which short-chain polyether groups have been attached.
The Electrochemical Society, Inc. publication, Volume 93-1, May 16 to 21, 1993 recites on page 2439 an abstract titled "Synthesis And Properties of New Polymer Electrolytes". The abstract refers to the preparation of functional polymer electrolytes including polymers having multiple functionality ion complexing sites, redox-active sites and immobilized anions, perhaps strategically placed as pendants or at chain ends.
U.S. Pat. No. 5,294,501 discloses a siloxane acrylate monomer and solid electrolyte derived by the polymerization thereof. Specifically, the disclosure is directed to siloxane acrylate and to a single phase solvent-containing solid electrolyte having repeat units derived from siloxane acrylate incorporated into the solid polymeric matrix of the solid electrolyte. An electrolyte cell that incorporates the electrolyte is also disclosed.
U.S. Pat. No. 5,061,581 disclosed an amorphous ionically conductive macromolecular solid having improved ambient temperature ionic conductivity. The solid comprises a solid solution of at least one positively charged ionic species dissolved in a macromolecular material, the macromolecular material comprising a polymer or copolymer having mostly a polyether structure. Some of the repeat units have the oxygens replaced with S or NR wherein R includes at least one basic site capable of associating with positively charged ionic species and has two to ten carbon atoms in the backbone.
To date, there are still problems limiting the use of polymer electrolytes in high energy density batteries (e.g. lithium--solid polymer electrolyte cells), most particularly in obtaining sufficient ambient temperature conductivity to permit attractive power densities (&gt;100W/l). For example, some of the most promising electrolytes are those based on poly(ethylene oxide)-salt complexes. PEO has shown good stability, a wide electrochemical stability window, and exhibits good solvating power for alkali-metal salts. The ambient temperature conductivities, however, are limited by the tendency of PEO-salt complexes to form crystalline phases. Since these phases have substantially lower conductivity than the amorphous material, the overall conductivity is reduced.
One of the approaches commonly used to improve the conductivity of PEO and other polymer complexes is to incorporate plasticizers (solvents) in the polymer electrolyte. These materials may reduce or eliminate the crystallinity in the polymer matrix and/or enhance the solubility of the salt. Suitable solvents are typically polar aprotic organic liquids, and among those well-known in the art are propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, dimethoxyethane (glyme), diglyme, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like. Other plasticizers that have been reported are oligomeric or low molecular weight polymeric materials such as poly(ethylene glycol).
In order to obtain significant improvement in conductivity (sufficient to approach that of liquid electrolytes), significant fractions (on the order of 20 weight percent or more) of such plasticizers are typically incorporated into the polymer electrolyte. At these concentrations, there is usually a degradation of the desirable mechanical properties and dimensional stability of the polymer matrix. Also the reactivity of the plasticizer will have a strong influence on the overall chemistry of the electrode, thereby negating some of the desired advantages postulated for all-polymer electrolytes.
It is an objective of the present invention to provide novel macromolecular materials having dendrimer structures which can be advantageously employed as a component of an electrolyte composition for application in electrochemical devices. By way of example, electrolytes formulated using the dendrimer structured materials of the invention, may have improved chemical and thermal stability and reduced volatility in comparison with conventional polar aprotic organic-liquid-based electrolytes.
Another objective of the invention is to provide a means of obtaining polymer electrolytes that are completely or predominantly amorphous at room temperature without requiring that a substantial fraction of the total composition comprise relatively low molecular weight organic solvents or plasticizing anions. By way of example, electrolytes formulated using the dendrimer- structured macromolecular materials of the invention may have higher room temperature ionic conductivity than conventional solvent-free polymer electrolytes, at least partially because of reduced crystallinity.
Another object of the present invention is to provide a novel polymeric macromolecular material having good conductivity properties so that it can be easily assembled as a component of a solid electrolyte cell.
The foregoing and additional objects will become more fully apparent from the following description.